gene6029

I often read about someone" lapping a piston " & never really quite understood the process. What exactly is involved in this work? Thanks, Gene

ffkiwi

Simply put-it means making a lap-which is made of a soft material-eg aluminium, with a lapping compound-similar to grinding paste- embedded in it, and working it over the piston whilst rotating the piston in a lathe,(by means of a holder-usually in the form of a dummy conrod) or the reverse for a cylinder lap-where the lap is turned by the lathe and the cylinder stroked up and down over the length of the rotating lap, usually in a figure of 8 pattern. In the case of a cylinder lap, the lap can be given a taper, normally in ten thousandths of an inch per inch of length, to assist in tapering the liner. Piston laps-which are much shorter-only about the thickness of the piston skirt are generally machined parallel. The intent-if home building parts-is to remove all machining marks from the piston, cylinder or crankshaft (yes you can lap shafts-but one seldom reads of it being done) and finally sizing them appropriately, to the right fit-the final process being completed by running in the engine (or component if a replacement part). We are talking of removing tenths of a thousanth of an inch, in terms of material removed. Various grades of lapping paste are available, and it is usually made into-or supplied as a paste with oil. Similar compounds rouge, diamond paste etc are used in jewellery and lapidary. Valve grinding paste-as used on full sized car engines-is a coarse grade of lapping paste. Laps are generally made adjustable-either by a tapered screw to expand them, in the case of a cylinder lap, or by a hoseclip or similar form of tightening device if a piston lap. Generally only used by home constructors these days, as commercial engine pistons and cylinders are generally finished by grinding or honing, and plain bearing engine shaft bearings by honing.

'ffkiwi' (occasional user of laps.....)

Jim Thomerson

With a TD, you take the glow plug off and put a prop on. I use Dupont white buffing compound, which is water based. put a little around the side of the piston and begin to slowly work the piston up and down. Wash the compound out when it turns grey and put new in. Work the piston up to about 1/16 inch of the top of the stroke. Leave that area to run in. Wash the engine thoroughly, oil it up and then run it. In times past the idea was that the hard piston would have tiny chips break off and imbed in the softer cylinder. These chips would then wear on the piston. Lapping supposedly got rid of the chips. I'm not sure anyone laps TD's anymore. I lap other iron and steel engines the same way. I think this puts a slight taper on the top edge of softer pistons, which I understand is a good thing.

gene6029

Thank you for the input. So if i understand correctly, any lapping is done at room temperature,ie the cylinder doesnt have to be at a running temp to lap the piston ? Once again thanks....Gene

combatpigg

My Feedback: (3)

The old iron / steel engines needed lapping in some cases, like a Fox .35 for example. Generally, it just isn't done unless the piston has high spots that are grabbing on the liner.
Lapping a modern engine will more than likely wear it out prematurely.
The original P/L fit on all my old TDs couldn't be tight enough for my taste.
Lapping the crankshaft is a different story, lots of Cox and Norvels needed it to put out max power. Cleansing powder and oil mixed into a paste. Sonic clean the parts afterwards.

gene6029

Thanks guys for the feed back....Gene

MJD

My Feedback: (1)

CP - how does one determine if the crank needs it or not - is it a matter of how long it will freewheel with the conrod off? I would guess you'd check that with some low visc oil, and would you use a prop or just freewheel the crank?

MJD

lfinney

My Feedback: (44)

does the engine leak a lil oil when running, if not i use red rouge to lap crank to case, they gotta leak some, like harley's &chicks[8D] or they aint running right.. i check them with some wd-40 after lapping, should spins smoothly

MJD

My Feedback: (1)

Harleys, chicks, and all British engines, esp. aero engines..

I have a tub of 2000 grit Aqua Buff compound - too coarse I would imagine? I think red rouge is finer yet. In fact, nano-"red rouge", or nano iron oxide is a high-zoot lens polishing compound. The stuff is amazing, each particle contains an average of 600 iron atoms and 900 oxygen. That is small. Not sure what the grit number would be, but it ain't four digits.

MJD

gcb

It would seem to me that the aluminum crankcase would always lose to the steel crankshaft in this case. I think most remove the crankshaft and polish it separately, then check fit to case. Some cases have an anodized surface to lessen wear which would quickly be damaged if lapped.

Still...you can't argue with success.

Sometimes you can check for a tight shaft by feeling the crankcase housing under the crankshaft. It will run hot if too tight.

George

combatpigg

My Feedback: (3)

MJD, with just a prop bolted to the crank and nothing else flip the prop. It should of course spin freely for a few seconds and slowly come to a stop...even allow the prop to reverse itself before it finally stops. If the crank doesn't come to a gradual stop, then it's still too tight. It should behave almost like a prop on a balancer. I use model fuel or MMO as a lube, sometimes water. The best lapping compound I've found is dry toothpaste.
This is the heart of all Cox builds, the best upper end parts won't perform to their full potential until the crankcase and shaft have been perfected. It's tedious because you have a lot of lap, clean & try cycles to go through sometimes before the fit is perfected. Never leave out the final sonic cleaning afterwards.

MJD

My Feedback: (1)

Cool, thanks. I own a lot of Cox engines and lost/destroyed a few too. But I've never done much more than a mix and match of parts to get what I need, or replace something fubar. Or clean out varnish too of course.

MJD

combatpigg

My Feedback: (3)

Back when I used to fly three or four 1/2A contests per year, and combat every Tuesday night down at the park, I always tried to have 4 engines ready to go out of a core group of 6. So mixing and matching was done, but that only produced 2 outstanding specimens. The rest had to come from sweat equity. Luckily back then the engines came borderline too tight which gave you something to work with.

Mr Cox

Lots of good advise here, I just wish I had an engine that needed lapping....

gene6029

Great information! Im glad i asked, answered alot for me.....Gene